Waveguide impedance transformers



Jan. 16, 1968 B. F. COOPER 3,364,383

WAVEGUIDE IMPEDANCE TRANSFORMERS Filed Aug, 19, 1963 2 Sheets-Sheet 1PRIOR ART I E P 4 2 4 Li H 'NV-EN-TOE ATTORNEYS Jan. 16, 1968 B. F.COOPER 3,364,383

WAVEGUIDE IMPEDANCE TRANSFORMERS Filed Aug. 19, 1963 2 Sheets-Sheet 2INVENTOR 64m We @071 BY 6am 4% w,

ATTOQN EY- 3,364,383 WAVEGUIDE IMPEDANCE TRANSFORMERS Brian FrederickCooper, Chelmsford, Essex, England, as-

signor to English Electric Valve Company Limited, London, England, aBritish company Filed Aug. 19, 1963, Ser. No. 303,054 Claims priority,application Great Britain, Oct. 19, 1962, 39,631/ 62 9 Claims. (Cl.315-39) This invention relates to waveguide impedance transformers suchas are often used, in microwave technique, for such purposes as couplingthe output resonator of a microwave oscillator, such for example as amagnetron, to its output waveguide. More specifically the inventionrelates to waveguide impedance transformers of the quarter-Wave type.

The invention is illustrated in and explained in connection with theaccompanying drawings in which:

FIGURE 1 is a diagrammatic representation of the prior art quarter-wavetype of impedance transformers to which this invention relates;

FIGURE 2 is an outer end view of a rectangular waveguide having acircular flange within which is mounted one illustrative waveguideimpedance transformer of this invention;

FIGURE 3 is an elevation view of the waveguide and waveguide impedancetransformer of FIGURE 2 partially in section;

FIGURE 4 is an inner end view of the waveguide and waveguide impedancetransformer of FIGURE 2;

FIGURE 5 is an elevation view of a slotted block employed in thewaveguide impedance transformer of this invention;

FIGURE 6 is an end view of the slotted block of FIGURE 5;

FIGURE 7 is a plan view of the slotted block of FIGURES 5 and 7;

FIGURE 8 is a perspective view partially in section showing theembodiment of FIGURES 2, 3, and 4 attached to the output of a microwaveoscillator; and

FIGURE 9 is an equivalent diagrammatic representation of the embodimentof FIGURES 2, 3, and 4.

Referring to FIG. 1 the output resonator of a microwave oscillator, forexample a magnetron, is represented by the block 1, the output of theresonator being indicated by dots. A load (not shown) is fed through amain waveguide 2 via an intermediate quarter-wave length ofwaveguide-commonly termed a transformer waveguide 3. The external Qvalue of the resonator 1 is determined by the impedance Z presented bythe load at the resonator output. If Z is the characteristic impedanceof the transformer waveguide 3, which is one quarter of a wavelengthlong, and if the main waveguide 2 is matched and is of characteristicimpedance Z the value of Z at the resonator output is given by:

Since, therefore, the external Q value of the resonator depends on thevalue of Z it is important, for eflicient and satisfactory operating, topre-determine this value accurately. It. is also desirable to allowadjustment in coupling to compensate for other dimensional variations.In known practice the waveguide 3 is accurately machined to determineits dimensions with precision. This is difficult and expensive to do,having regard to the high degree of accuracy commonly required. Thepresent invention seeks to provide improved waveguide impedancetransformers of the quarter-wave type without this disadvantage andwhich shall be such that the transformer waveguide which forms part ofthe transformer can be i United States Patent 0 easily adjusted, aftermanufacture. In this way cost of manufacture is materially reduced.

According to this invention a quarter-wave type waveguide impedancetransformer includes .a transformer waveguide which is deformable topermit alteration of the characteristic impedance thereof to adjust thesame accurately to a desired pre-determined value.

Referring to FIGS. 2 to 8 inclusive, these shown an embodiment of thisinvention wherein the main waveguide 2 of characteristic impedance Z isa rectangular waveguide one broad wall of which appears in FIG. 3 inwhich the guide is shown broken away. The said main guide terminates ina rectangularly apertured circular flange member 4 which is adapted tobe fitted to the output resonator of a magnetron or other microwavedevice. Normally the space in the resonator forms part of the evacuatedspace of the device and, in order to preserve the vacuum, a glass orother suitable vacuum-tight window (not shown) may be sealed across thecross-section of the guide 2 near the flange member 4 in accordance withcustomary practice. In the end of the guide 2 where it enters the flangemember 4 are fitted two similar rectangular metal blocks 5. One of theseblocks is shown separately in FIGS. 5 to 7. In the construction shownthey are made separately from the guide 2 and flange member 4 but arefitted in the mouth of the guide 2 and brazed to the opposite broadwalls thereof so as to be, structurally speaking, part of the guide. Oneexternal dimension x of each block 5 is the same as the larger dimensionof the interior cross-section of the guide 2; a second dimension y ofeach block is equal to one quarter of the intended working wave length;and the third imension z is of such value that 2z is less than thesmaller dimension of the interior cross-section of the guide 2. Eachblock 5 has a slot 6 of length less than one quarter of a guided wavelength cut in it parallel to and close to one of the two faces which areof dimensions x by y, the slot running the full length of the dimensionx. The blocks 5 are fixed in the mouth of the guide 2 with their slottedfaces facing into the guide 2 and the slots 6 near the space which isleft between the blocks. The arrangement is best shown in FIGS. 4 and 8.The quarter-wave transformer waveguide 3 is constituted by the spacebetween the blocks 5 and it will be seen that, in the constructionshown, the cross-section of the guide hasone dimension x and the otherdimension, which will hereinafter be termed the height dimension, equalto the separation between the adjacent faces of the blocks 5. Thisdimension, the height dimension, can be readily adjusted over a rangewhich is adequate for practical purposes, by inserting a suitable toolinto the waveguide 2 (before, of course, the window-if anyis fittedtherein) and widening the slots by inserting the tool therein or, ifdesired, narrowing the slots by inserting the tool between the blocks 5.This results in flexing or bending the thin portions 5a of the blocksleft between the slots 6 and the adjacent faces parallel thereto,thereby altering the separation of the said faces and with it the heightof the transformer waveguide and thereforethe value Of Z1.

Preferably the adjustment is made such that the height of thetransformer guide 3 is constant (at any cross section) over thedimension x. This requires that the slots 6 shall extend for the fulllength x as is the case in the illustrated embodiment. This is, however,not essential and the slots can be shorter giving adjustment over ashorter length and non-constant height for the transformer waveguide 3over a given cross section. As will be seen from the drawings the slotsform two short-circuited waveguides which are not cut off at thetransformer design frequency. The design must, of course, be such thatthey are of acceptably small electrical effect on the performance of thetransformer. This requirement, and how it may be satisfied, will bebetter understood from FIG. 9, which is an approximately equivalentdiagrammatic representation of the illustrated embodiment.

Referring to FIG. 9 this equivalent diagram comprises the resonator 1,the transformer waveguide 3, the main waveguide 2 and two branch guides66 constituted by the slots 6. If the branch waveguide has a lowattenuation constant and its characteristic impedance is Z then Z theimpedance presented by the series branch waveguide at its mouth (seeFIG. 9) is given by:

Z =Z tan a 1 where on is the phase constant and 1 is the dimension shownin FIG. 9 (2) If a and 11 are, respectively, the broad and narrow crosssectional dimensions of the main guide 2 and a and 11 are respectively,the broad and narrow crosssectional dimensions of the branch guide 66.

If the branch guide is approximately one quarter of a wave length longand a (approx.)=a

Z (approx.)=Z,Z:

It will be seen therefore, that if the narrow cross sectional dimensionsof the branch wave guide-i.e. the width of the slot 6-is kept small inrelation to the narrow cross-sectional dimension of the main guide 2,the normalised impedance at the mouth of the branch waveguide will besmall and the said branch waveguide will have only small effect on theimpedance presented by the load (feed via the guide 2) at the terminalsof the resonator 1.

While one form of the invention has been shown for purposes ofillustration, it is to be understood that various changes in the detailsof construction and arrangement of parts may .be made without departingfrom the spirit and scope of the invention as defined in the appendedclaims.

I claim:

1. A quarter-wave impedance transformer including a quarter-wave lengthof rectangularly sectioned guide having two opposite broad faces of thinmetal formed of thicker metal portions incompletely slotted to a depthof substantially one-quarter of a wave length and defining said thinmetal faces supported by bendable necks of metal carrying said thinmetal faces.

2. A transformer according to claim 1 wherein said broad face formingthicker metal portions each comprise a metal block, said broad facescomprising broad faces of said blocks, the slotting of said thickermetal portions being parallel to said broad faces and extending lessthan fully through each block, said bendable neck being defined by theunslotted portion of said blocks adjacent the termination of theslotting within said blocks.

3. A transformer according to claim 1 wherein the slotting of saidthicker metal portions extends fully along one dimension of said thickermetal portions.

4. A transformer according to claim 1 for use in association with a mainwaveguide in communication therewith, the width of the slotting of saidtricker metal portions being substantially less than the width of themain waveguide.

5. A magnetron structure comprising in combination a magnetron and aquarter-wave impedance output transformer as defined in claim 1.

6. A magnetron structure comprising in combination a magnetron and aquarter-wave impedance output transformer as defined in claim 4.

7. A quarter-wave impedance transformer comprising a quarter-wave lengthof rectangularly cross-sectioned wave guide, a pair of metal portionshaving opposed broad face portions comprising opposed broad faces ofsaid quarter wave length of wave guide, each of said metal portionshaving a slot therein in proximity to said face portions to definerelatively thin Walls intermediate said broad faces and said slots, andneck portions interconnecting said thin walls and the remainder of saidmetal portions, said thin Walls being adjustable toward and away fromeach other for adjustment of the characteristic impedance of saidquarter-wave impedance transformer.

8. A quarter-wave impedance transformer according to claim '7 whereineach of said slots comprises a short circuited wave guide.

9. A transformer according to claim 7 wherein the depth of said slot issubstantially one-quarter of a Wave length.

References Cited UNITED STATES PATENTS 2,411,534 11/1946 Fox 333-352,432,093 12/1947 Fox 333--35 2,466,922 4/1949 Wax 315-39 2,526,39910/1950 Ohress 333-35 2,531,437 11/1950 Johnson 333-35 2,576,186 11/1951Malter 33335 2,607,849 8/1952 Purcell et al 33335 2,666,869 1/1954Clogston et a1. 3l5-39 2,701,343 2/1955 Lange 333-83 2,805,337 9/1957Dunsmuir 315-3953 HERMAN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

1. A QUARTER-WAVE IMPEDANCE TRANSFORMER INCLUDING A QUARTER-WAVE LENGTHOF RECTANGULARLY SECTIONED GUIDE HAVING TWO OPPOSITE BROAD FACES OF THINMETAL FORMED OF THICKER METAL PORTIONS INCOMPLETELY SLOTTED TO A DEPTHOF SUBSTANTIALLY ONE-QUARTER OF A WAVE LENGTH AND DEFINING SAID THINMETAL FACES SUPPORTED BY BENDABLE NECKS OF METAL CARRYING SAID THINMETAL FACES.